This invention relates to lithium electrochemical power sources, and more particularly, to methods for making electrolytes for such power sources.
Electrolytes containing a mixture of tetrachloroaluminate salts dissolved in sulfur dioxide are especially useful for rechargeable cells in which the negative electrode is lithium metal and the positive electrode is a high surface area carbon. Lithium/metal tetrachloraluminate.cndot.nSO2/carbon rechargeable cells have been described in the patent and scientific literature, including Kuo et al U.S. Pat. No. 4,513,067 and the paper presented by A. N. Dey et al at the 32nd Power Sources Symposium at Cherry Hill New Jersey in May 1986 and reported in the Proceedings of the Electrochemical Society. The Kuo patent, for example, relates to batteries having an alkali or alkaline earth metal anode, a carbon cathode, and a sulfur dioxide electrolyte. Example 1 of the patent describes a basic battery including lithium tetrachloroaluminate dissolved in sulfur dioxide. Such cells typically contain pure lithium anodes, porous carbon cathodes, and lithium tetrachloroaluminate as the electrolyte salt dissolved in liquid sulfur dioxide in a molar ratio of LiAlCl.sub.4 to SO.sub.2 of 1 to 6. During discharge, the aluminum and SO.sub.2 take part, producing a product that adheres strongly to the carbon cathode resulting in the electrolyte having lower aluminum concentration.
More recently, batteries having improved capacity, resistance to capacity fading, and lithium plating morphology, were obtained by increasing the concentration of aluminum in the electrolyte. The molar ratio of aluminum salt to sulfur dioxide was changed from 1 to 6 to 1 to 3. To prevent the electrolyt mixture from freezing, either calcium or sodium tetrachloroaluminate was mixed with the lithium tetrachloroaluminate.
While solving the freezing problem, the concentration of hydrolysis products (impurities containing hydrogen as the result of contact with water) also increased. These hydrolysis products were suspected of contributing to these cells' increased propensity for rapid and dangerous reactions and explosions during charging, as well as other difficulties including the growth of lithium dendrites which are responsible for the formation of short circuits during charge cycles. This moisture comes into the system primarily with one of the component salts, namely, aluminum chloride, in spite of attempts to purify it by distillation or sublimation prior to electrolyte preparation.
Schlaikjer, U.S. Pat. No. 4,925,753, addressed the problem of moisture and discloses a lithium tetrachloroaluminate cell containing calcium tetrachloroaluminate antifreeze that is characterized as substantially moisture free. In accordance with the method described in this patent, the lithium chloride and calcium chloride component salts are dried in vacuo for at least 16 hours at least 200.degree. C., and the aluminum chloride is purified by distillation at atmospheric pressure from molten lithium tetrachloroaluminate in the presence of calcium turnings. This method was the best way applicants then knew to reduce the moisture content to a low level. For example, FIG. 1 hereof shows an infrared (IR) spectrum of 2LiCl.cndot.CaCl.sub.2 .cndot.4AlCl.sub.3 .cndot.12SO.sub.2, using a 10 mm pathlength type I quartz cell versus an identical empty cell, between 4000 and 2000 cm.sup.-1. The aluminum chloride had been distilled from molten lithium tetrachloroaluminate in the presence of calcium turnings as described in the Schlaikjer patent. The salt was free of organic materials and iron, but was still only 98 weight percent pure, as determined by using argentimetric titration. The lithium and calcium chloride salts were reagent grade and had been dried in vacuo at 200.degree. C. for at least 14 hours. The sulfur dioxide was Matheson reagent grade, with less than 50 ppm water. Nevertheless, the strong, broad absorption between 2.8 and 3.15 .mu. in FIG. 1 represents absorption caused by --OH stretching, believed to be present on the anion as AlCl.sub.3 --OH.sup.-. Although 98% AlCl.sub.3 represents "substantially moisture free" aluminum chloride, 2 weight per cent of water represents 13 mole per cent of the mixture with aluminum chloride. Still lower levels are desired for an electrolyte in a lithium cell. Accordingly, a main object of the invention is the provision of a process for preparing electrolytes that is a further improvement over the prior art.